noun
A fixed idea or bias formed before knowing the facts; often used in the phrase 先入主にとらわれる (to be bound by preconceptions).
先入主にとらわれずに、まずは話を聞いてみよう。
Let's listen to the story first without being bound by preconceptions.
彼の意見は先入主に基づいている。
His opinion is based on prejudice.
先入観 is a near-synonym also meaning preconception, but 先入主 is less common and may carry a slightly stronger nuance of a fixed, subjective viewpoint.
Compound of 先入 (sennyū, 'entering beforehand') and 主 (shu, 'master, main'). The exact historical derivation is uncertain, but the word conveys the idea of a preconceived notion that dominates one's thinking.